1. Field of the Invention
The present invention relates to a projecting zoom lens to be mounted on a projection type display apparatus and having a six lens group structure including three or four moving lens groups, and a projection type display apparatus with such projecting zoom lens mounted thereon.
2. Description of the Related Art
Recently, a transmission type or a reflection type liquid crystal display apparatus and a projector apparatus (a projection type display apparatus) such as a DMD display apparatus using a light valve have been widely spread and have been enhanced in their functions. Especially, with enhancement in the resolution of the light valve, there has been increasing a request to enhance the resolving power of the projecting lens.
Also, in order to enhance the freedom to set the distance between the projection type display apparatus and a screen as well as to enhance the possibility of installation of the apparatus in a narrow indoor space, there has been increasing a request to mount a higher-performance wide zoom lens or a high-power zoom lens onto the projection type display apparatus having a compact structure. In view of this, there have been made various proposals which aim at realizing such request (see JP-A-2001-350096, JP-A-2004-70306, and JP-A-2005-62226).
On the other hand, due to such enhancement in the resolution of the light valve and the resolving power of the projecting lens, there is raised a big problem that the properties of the light valve and projecting lens in the whole zooming area ranging from a wide angle end to a tele end are caused to vary. For example, in order to correspond to the enhanced resolution of the light valve, there is requested a fast lens having a large aperture so that light having passed through pixels may arrive at the screen without losing the amount of the light. However, generally, when the resolving power of the lens varies from the wide angle end to the tele end, the fastness (that is, Fno.) of the lens increases, which results in the dark screen.
Also, as a projecting zoom lens in which the number of moving lens groups of zoom lens is increased to aim at enhancing the optical performance in the whole zooming area from the wide angle end to the telephoto end, there is known the projecting lens that is disclosed in JP-A-2003-337283. However, in this lens, there is raised a problem that the variable amount of spherical aberration caused when the power of the lens is varied and, especially, in the tele end, the marginal rays are large to thereby provide an under-fastness tendency.
In view of this, as disclosed in JP-A-2005-84455, there is proposed a projecting lens in which, of 6 lens groups forming a zoom lens, in the third lens group, there is disposed a surface having a strong negative refractive power to thereby reduce the amount of variations in the spherical aberration caused when the power of the lens is varied.
As described above, there is found a problem that the properties of the lens are caused to vary according to the variable power: that is, when the projection distance is varied depending on the size of a projection space in a room, the image of an object cannot be projected on the screen with the same brightness; and thus, there has been increasing a request to take proper measures against this problem quickly.
Also, in the projecting zoom lens disclosed in the above JP-A-2005-84455, although the varying amount of the spherical aberration caused by the variable power is reduced, there is raised a problem that the properties of the lens are caused to vary according to the variable power: that is, the astigmatism of the lens increases.